Frequently, aircraft, tank and helicopter gas turbine engines are operated in a desert environment where the gas turbine compressor rotor blades and stator vanes are exposed to erosive media such as sand and dust. These erosion effects lead to increased fuel consumption, horsepower loss, higher overall operating temperatures, and can cause damage to compressor and turbine hardware. Erosion resistant coatings such as TiN, TiCN, TiZrN, TiZrCN, TiAlN and TiAlCN, applied by cathodic arc physical vapor deposition, can be used to prolong the life of compressor airfoils in a sand erosion environment. See, for example U.S. Pat. No. 5,071,693.
Physical vapor deposition is a line of sight coating process. Substrates to be coated need to be manipulated in the vapor to achieve uniform coverage. Sections of the substrate that do not require a coating need to be masked adequately.
Turbine compressor airfoils have an airfoil section which extends outward to pump the air into the combustion chamber. The portion of the turbine airfoil opposite the airfoil portion is used to attach the airfoil to the disk or rotor part of the engine, which is not in the flow of the air, and therefore is not in need of protection from erosive effects. This portion of the turbine airfoil often has the shape of a dovetail, which is assembled into dovetail slots on the disk or rotor portion of the engine. Hence, it is frequently referred to as the dovetail.
The walls of the dovetail portion of the turbine airfoil contact the walls of the dovetail slots of the disk or rotor. After a long period of time or rotating at high speeds, the dovetail walls exhibit a fatigue-related phenomenon referred to as fretting. Fretting has been found to be exacerbated by coatings applied to the airfoil portion of the turbine airfoil. Thus, in order to achieve the desired properties in the various portions of the turbine airfoil to maximize the life of the turbine airfoil, it has been necessary to devise methods to properly coat the airfoil portion of the turbine airfoil without affecting the dovetail portion of the turbine airfoil.
Physical vapor deposition is extensively used to apply ceramic thermal barrier coatings on turbine airfoils by electron beam evaporation. Extensive work has been done to design processes and fixturing to apply a uniform coating on a variety of airfoil sizes and shapes.
U.S. Pat. No. 5,997,947 discloses a modular, rotisserie type coating fixture for use in electronic beam physical vapor deposition (EBPVD) coating processes. The fixture includes a support structure and means attached to the support structure for allowing it to be rotated about a first axis. The fixture further includes cassette means within the support structure for holding one or more workpieces to be coated. The cassette means are joined to the support structure by spindles which allow the cassette means to rotate about a second axis substantially parallel to the first axis and thereby allow each workpiece being coated to rotate about its longitudinal axis. The cassette means support each workpiece so that surfaces of the airfoil to be coated are maintained substantially parallel to the coating source.
As indicated above, cathodic arc physical vapor deposition can be used to apply erosion resistant coatings. Cathodic arc physical vapor deposition is, in the first order, a line of sight deposition process. Vapor created by the cathodic arc discharge on the cathode surface propagates basically in a straight line from the cathode surface towards the workpieces. Second order effects in vapor propagation include scattering of the ionized vapor in the chamber gas atmosphere, which creates some throwing power around corners and into cavities and more significantly, electrostatic attraction of the highly ionized vapor, and thus enhanced coating build-up on edges and sharp contours of the workpieces such as airfoil tips and leading and trailing edges of the airfoils. The predominant deposition feature however, when coating the surface of flat workpieces such as the platform and airfoil of compressor blades and vanes, is the line of sight propagation of the vapor. To provide the maximum coating thickness on the workpieces such as the lower part of the airfoil and on the platform, the view between the cathode surface and the workpiece surfaces should not be obstructed.
There continues to be a need in the art for coating fixtures for use in a physical vapor deposition coating operation which allow for simultaneous coating of a plurality of workpieces and which promote the production of high quality coatings. The coating fixtures should permit coating application to only those portions of the workpieces requiring protective coatings while protecting those portions not requiring a protective coating. Further, the coating fixtures should be reusable and relatively inexpensive to fabricate.